1. Field of the Invention
The present invention relates to threaded connections for securing together the ends of tubular bodies. More specifically, the present invention relates to a threaded connection for connecting together tubular pipe bodies used in the construction of wells.
2. Description of the Prior Art
The pipe used in drilling and completing oil and gas wells and other wells employed in extracting minerals from the earth is typically in the form of a long string of pipe segments secured together by threaded connections provided at the end of each pipe segment. The connections serve the dual function of holding the adjoining segments together and providing a pressure seal at the connection. Stresses are encountered in the threaded connection that result from forces applied to the connection during its initial assembly into a string, forces associated with the placement of the string into the well and forces resulting from the pressure differentials acting across the engaged connection. When the stresses acting on the connection are either excessive or inadequate, the connection can fail, causing separation of the string or disrupting the pressure seal within the connection.
During the makeup and subsequent running and utilization of a typical string of casing or tubing employed in a well, the forces acting on the connection may alternate between high tension and high compression forces. Tension forces are imposed, for example, when the connection is part of a heavy string that is suspended from the well surface. Compression forces occur during the makeup of the connection and during the process of placing the connection into a well where the well bore is deviated such that the pipe string must be bent around a curve in the well bore. In the latter situation, the portion of the connection at the inside of the curve is stressed in compression relative to the portion of the connection on the outside of the curve. Concentration of excessive compressive forces within the connection can cause the connection to be permanently deformed or to fail when the compressive forces exceed design limitations of the connection.
Current drilling and completion applications require increasing compression ratings for strings being used in deeper and more deviated wells. The well string designs for these critical conditions often require the use of connections having external radial dimensions that are the same as those of the pipe or only slightly larger than the pipe body. Reducing the volume of material employed in forming the connection in an effort to reduce the pipe diameter increases the stress required to be sustained by the remaining material of the connection. In general, these reduced diameter connections have a low elastic compression rating as compared with the connections having larger outside diameters. Use of the smaller diameter connections in critical wells increases the probability that the connection will be exposed to stresses that exceed the elastic limits of the elements of the connection. Exceeding the elastic limit of the components of the connection changes the characteristics of the connection, which increases the likelihood of failure of the connection.
Conventional threaded connections fall generally into the category of interfering or non-interfering, or a combination of both. Threads that do not interfere are sometimes referred to as xe2x80x9cfree running.xe2x80x9d A connection having interference threads has dimensions such that the threads of one component interfere with the threads of the adjoining component to cause a mechanical deformation of the material of the engaged threads. Threads in a free running, non-interference-type connection may be engaged without causing any mechanical thread deformation in the made-up connection.
Some thread connections may include a combination of both interference and non-interfering, or free-running, threads. An important component of the makeup of a free running thread connection is a mechanical limit, such as a torque shoulder, that permits the connection to be tightened. In many cases, the torque shoulder also provides a sealing surface between the engaged pipe sections. Some prior art designs using free running threads provide a radial seal adjacent the torque shoulder by tapering the internal surfaces extending to the torque shoulder and forcing the tapered surfaces together during the makeup. The sealing between interference fit threads is normally obtained by mechanical engagement of the threads assisted by a void filling thread compound.
FIG. 1 illustrates a conventional prior art connection using two-step, free-running threads 11 and 12 separated by a central torque shoulder 13. The torque shoulder is formed by the engagement of circumferential shoulders in each of the members of the connection.
FIG. 2 illustrates details of the torque shoulder of the connection of FIG. 1. A typical shoulder area of a threaded connection with free-running threads has radial gaps 14 and 15 between the surfaces of the female component of the connection, or the box 16, and the male component of the connection, or the pin 17. The gaps 14 and 15 result from the large machining tolerances permitted in order to make the manufacture of the connection easier and less expensive. The presence of the gaps 14 and 15 also contributes to the ease of assembly of the connection.
The torque shoulder 13 is formed in the engaged contact area indicated at 18. The radial dimension of the contact area 18 is less than the radial dimension of the respective elements of the torque shoulder formed on the pin and box sections by an amount equal to the radial dimension of the gap 14 or 15. In some conventional connections, the area of contact represented by the bearing surface 18 may be as little as 70% of the total available surface area of the torque shoulder.
In a connection such as illustrated in FIG. 2, the compressive forces exerted against the torque shoulder during the makeup or other compressive loading of the connection can cause the areas of the torque shoulder with the smallest cross-sectional dimensions to be plastically deformed as indicated in FIG. 4. The plastic deformation is accommodated in the gaps 15 and 14 adjacent the torque shoulder 13. The deformation of the pin shoulder is indicated at 19, and the deformation of the box shoulder is indicated at 20. FIG. 4 illustrates that, under the influence of compression loading, the corners of the torque shoulder 13 will flex and distort into the open radial gaps, which permits yielding to occur at a load that is less than that theoretically sustainable by a torque shoulder having full engagement of the contacting surfaces of the torque shoulder 13. As may be appreciated, a connection such as illustrated in FIGS. 1-4 is limited in its compressive capabilities to the compressive forces that cause yielding of the weakest point of the torque shoulder that occurs at the unrestrained smallest cross-sectional area at the outside, extreme corner of the torque shoulder.
The thread design of the conventional connector illustrated in FIGS. 1-4 also plays a part in the compressive strength of the connection. As illustrated in FIG. 3, in such connections a gap 21 exists between the stab flanks of the threads 22 of the pin 17 and threads 23 of the box 16. As with the gaps formed about the torque shoulder 13, the gap 21 results from large machine tolerances that contribute to simplifying the manufacture and assembly of the connection. When the connection becomes sufficiently loaded in compression, the gap 21 is closed and the stab flank of the threads can begin to share the compressive loading exerted on the torque shoulder. However, the degree of applied compressive force necessary to close the gap 21 can exceed that required to produce the deformation of the torque shoulder indicated in FIG. 4. The net result is that the compressive loading rating for the connection illustrated in FIGS. 1-4 is limited to a value below that which would produce the yielding of the thinnest, most vulnerable portions of the torque shoulder.
The compressive load-bearing surface area in a threaded connection is increased to increase the stress compression capacity of the connection. The increased surface area is obtained by increasing thread contact areas at lower compressive loadings and by increasing the engaged area of a torque shoulder provided in the connection. The shoulder area increase is obtained by tightening the machining tolerances used in making the connection. The increased surface contact area of the torque shoulder reduces the force per unit area and limits clearance for receiving plastic deformation of the shoulder, which creates a stiffer shoulder that reduces shoulder flexing and distortion. The increase in thread dimension reduces the spacing between the stab flanks of the threads so that the gap between engaged pin and box threads closes at lower compressive loading, allowing the threads to share compressive loads with the torque shoulder before the torque shoulder is yielded.
From the foregoing, it will be appreciated that a primary object of the present invention is to increase the torque loading rating of a threaded connection employed to secure well tubulars together.
Another object of the present invention is to increase the compressive loading rating for a tubular connector without increasing the external dimensions of the connector.
An object of the present invention is to increase the contact area of a torque shoulder in an engaged threaded tubular connection to reduce the compressive loading on the torque shoulder.
Yet another object of the present invention is to provide an increased compressive load rating for a free-running thread configuration employing a torque shoulder in which the confined area about the engaged torque shoulder is limited to reinforce the torque shoulder along its area of smallest cross-sectional dimensions to prevent plastic deformation of the torque shoulder into voids adjacent the vulnerable area of the torque shoulder.
Another object of the present invention is to provide a connection having a pin and box connector member employing a torque shoulder wherein the stab flanks between the threads of the adjoined connector members are reduced to the maximum possible whereby the thread stab flanks will engage to assist in distributing compressive forces before such forces exceed the yield limitations of the torque shoulder.
The foregoing, as well as other, objects, features, and advantages of the present invention may be more readily appreciated and understood by reference to the following drawings, specification, and claims.